Poinsettia Production Problems and Disorders

Roberto G. Lopez, Ph.D., Assistant Professor & Floriculture Extension Specialist, Purdue University

Poor and Uneven Branching
Poor branching is a disorder that results in uneven lateral shoot breaking (top breaks are larger and stronger than bottom breaks, Figure 1) or lack of breaks on branched plants. Often growers wait too long to pinch their plants resulting in poor and uneven branching. In such instances, plants become tall and lateral shoots emerge before pinch and break unevenly after pinch. For most cultivars, 14 days is the maximum recommended time from planting to pinching. Certain cultivars are more susceptible to disorder and a reduction in time to pinch will minimize irregular branching. In addition, production temperatures >75 °F during growth can cause blind shoots, or reduced branching. Uniform poinsettia branching can be achieved with proper plant spacing, pinch timing and technique, and temperature management.

Leaf Curl and Desiccation
Symptoms of leaf curl, scorch or desiccation typically occur on leaves in the middle portion of the plants. As your poinsettia crop is actively growing when greenhouse temperatures and light levels are high, it is essential that you provide adequate irrigation to avoid extreme conditions (excessively dry or wet) that can result in leaf desiccation (Figure 2), poor growth, and disease susceptibility. Excessively dry plants should not be irrigated with cold water as uptake and transport by the roots can be inhibited for a short interval. Under extended drought stress, lower leaves will turn yellow and senesce.

Leaf Distortions
Poinsettia leaf distortions (Figure 3) often occur on young or immature leaves and are believed to be caused by many factors. Physical damage during pinching, ruptured cells and latex residue can cause the expanding leaf to become distorted. Environmental stresses, overhead fertilization with phosphorus fertilizers during propagation or production, abrasion or thrips may lead to distorted leaves. Leaf distortion can also results from dramatic temperature and humidity changes as plants are moved from propagation to production. Typically, as plants grow, mature leaves will cover the distorted or damaged young leaves and not influence the marketability of the crop.

Split Bracts
Split bracts (Figure 4) occur when a poinsettia plant is exposed to a period of short days (malfunctioning day extension or night interruption lighting, stock plants exposed to short days) that lead to flower initiation followed by long days. Cool night temperatures in late September can cause flower initiation and a subsequent period of very hot nights can induce vegetative growth, leading to split bracts. Certain cultivars can be more susceptible to split bracts than others.

Poinsettia Diseases - Pythium

Roberto G. Lopez, Ph.D., Assistant Professor & Floriculture Extension Specialist, Purdue University
Janna Beckerman, Ph.D., Assistant Professor & Plant Pathology Extension Specialist, Purdue University

Pythium Root and Stem Rot

Pythium root and stem rot (Pythium spp.), sometimes called black leg, is a disease that is spread easily by water and can occur any time during the propagation and production of poinsettia. The first appearance of Pythium commonly occurs during the rooting of cuttings, where the wound provides an easy infection court for Pythium. Symptoms of Pythium at this stage can be easily confused with Rhizoctonia root rot, or Erwinia soft rot. Be sure your diagnosis is correct as the fungicides that effectively control Pythium are distinct from the fungicides that control Rhizoctonia!

Pythium infections can develop yet again at the end of production when conditions are ideal (cooler temperatures, overcast weather, poorly drained and excessively wet media). It is also important to remember that fungus gnats can disseminate Pythium and other soilborne pathogens, and should be closely monitored and managed. Symptoms of Pythium infection late in production include wilting; chlorosis of the mid-vein region and lower foliage (Figure 1); stunting; black or brown cankers at the soil line; blackened, mushy roots, and eventual plant death. It is important to periodically check the roots of your plants even if none of the above symptoms are observed. As with most root rots, the outer surface of roots infected with Pythium will typically slide off exposing a thread-like root (Figure 2). Once symptoms are observed, infected plants should be immediately discarded and care should be taken to avoid scattering debris and media of infected plants.

Managing Pythium requires an integrated approach. As Pythium is a water mold, water management is essential, and the moisture holding capacity of the potting medium is critical. High moisture holding media (like highly decomposed peat) results in worse Pythium root rot compared to high quality peat that is not greatly decomposed. The incorporation of biologicals (RootShield, MycoStop, RootGuard) assists in the management of disease when the infection level is low. Research has found that the incidence and severity of the disease is worse when the pH of the medium is above 5.5. Studies have also shown greater disease severity as Nitrogen levels increase from 100 ug N/g to 600 ug N/g.

Several fungicides are labeled for control of Pythium root and stem rot. As with all pesticides, it is important to rotate between chemical classes to prevent fungicide resistance. Table 1 provides fungicide drenches that are recommended for the prevention of pythium on poinsettia (Always consult labels carefully for exact rates and to see if the material is registered in your state). Reference to fungicides is supplied with the understanding that no discrimination is intended and no endorsement is implied by Purdue University.

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Figure 1. Poor and uneven branching

Figure 2. Leaf curl and desiccation

Figure 3. Leaf distortions

Figure 4. Split bract

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Figure 1. Wilted poinsettia foliage due to pythium root rot

Figure 2. Poinsettia with pythium root rot